As well known to those skilled in the art, photonic crystal fibers are classified into high refractive index core-typed photonic crystal fibers in which a clad having hollows covers a high refractive index core, and light guidance is accomplished according to a total internal reflection mechanism, and low refractive index core-typed photonic crystal fibers in which a clad covers a low refractive index hollow core and light guidance is accomplished according to a photonic crystal band gap mechanism.
Light guidance properties of a photonic crystal fiber having a clad layer in which a photonic crystal structure is formed are disclosed in many articles. For example, Knight et al., “All Silica Single Mode Optical fiber with Photonic Crystal Cladding”, Optics Letters, V. 21, No. 19, p 1547 (1996), and Birks, et al., “Endless Single Mode Photonic Crystal Fiber”, Optics Letters, V.22, No. 13, p. 961 (1997) disclose characteristics of a single mode photonic crystal fiber comprising a silica core and a porous silica clad layer. In these technologies, a silica tube positioned at a center of a bundle of triangular lattices consisting of a plurality of silica tubes is substituted with a silica rod and the resulting structure is drawn at a high temperature to fabricate the photonic crystal fiber. The silica rod corresponds to the core of a conventional step-index photonic crystal fiber. The clad layer which covers the silica rod has hollows periodically arranged in a transverse section of the photonic crystal fiber. Hollows in the clad layer are axially extended to both ends of the photonic crystal fiber, and so an effective refractive index is changed in the clad layer, thereby causing light guidance properties such as a diameter of mode field or dispersion property to be changed. A mode power distribution in the transverse section of the photonic crystal fiber is one of the most important factors in determining physical properties of the photonic crystal fiber, and a change in the effective refractive index of the clad layer changes the light guidance properties of the photonic crystal fiber. Furthermore, a high power laser beam can be guided in a single mode, differently from the step-index photonic crystal fiber, and advantageously, a relative refractive index of the clad layer against the core can be controlled by changing a hollow fraction in the clad layer. In these photonic crystal fibers, light guidance is accomplished according to a total internal reflection mechanism, i.e. a conventional light guidance mechanism, and the photonic crystal fiber comprises a core region with a high refractive index and a clad layer with a low refractive index. Physical properties of a conventional photonic crystal fiber are described in Broeng et al., “Invited Paper: A new class of optical waveguides”, Optical fiber Technology, V.5, p. 305 (1999).
Meanwhile, it is known in the art that when a core, covered with a clad layer, consists of hollows arranged in a predetermined periodical lattice such as a triangle or a honeycomb, a photonic crystal band gap is two-dimensionally formed against a light with a frequency corresponding to the above structure, and so the light is limited within the core region, thereby light guidance is accomplished (Birks et al., “Full 2-D photonic bandgap in silica/air structures”, Electronics Letters, V.31, No. 22, p. 1941 (1995), Knight et al., “Photonic band gap guidance in optical fiber”, Science, V.282, p. 1476 (1998)).
The photonic crystal band gap structure is considered as a novel notion, and in the case of a photonic crystal band gap material with a three-dimensional periodicity, light with a wavelength corresponding to the structure limitedly flows through the photonic crystal band gap material. In the case of a photonic crystal band gap material with a two-dimensional periodicity, a propagation constant β vertical against a periodic plane in which propagation is forbidden is within a predetermined range of values. The photonic crystal band gap material with a two-dimensional periodicity may be utilized to realize light guidance by limiting a light only within a hollow core region covered with the clad layer consisting of the photonic crystal band gap material. As described above, a photonic crystal fiber having a clad layer in which dispersed phases are desirably arranged causes light to be guided only within the core. Accordingly, this photonic crystal fiber may greatly contribute to a high-speed transmission of a large amount of data through an information communications net:
Physical properties of the photonic crystal fiber depend on a refractive index ratio of a continuous phase consisting of an optical material and a dispersed phase mostly consisting of air, a size of the dispersed phase, and a distance between centers of dispersed phases. Additionally, an existence of the photonic crystal band gap depends on these factors.
In a conventional photonic crystal fiber, silica with the refractive index ratio of 1.45 is used as the optical material constituting the continuous phase. The conventional photonic crystal fiber was prepared by tying silica tubes and a silica rod into a bundle in such a way that silica tubes and the rod are arranged in a predetermined manner and drawing them at a temperature of 2000° C. Production of the conventional photonic crystal fiber mostly depends on factors such as a size of each hollow, a distance between centers of dispersed phases, and an arrangement shape of hollows. Such factors cause an effective refractive index of the clad layer to change, and light guidance is realized due to a difference of the refractive index between the clad layer and a silica core, as disclosed in U.S. Pat. No. 5,802,236 and International Pat. Pub. Nos. WO 00-16141, WO 00-37974, WO 99-00685, WO 00-49435, WO 00-49435, WO 00-60358, WO 99-64903, and WO 00-64904.
However, a method of fabricating a photonic crystal fiber preform with a fine-structured section by extruding a polymer optical material with a refractive index of 1.45 is not well known in the art. An extrusion of these porous fibers is applied to fabricate an endoscope comprising multiple core hollows (U.S. Pat. No. 5,471,553) and to fabricate a photonic crystal fiber for optically transmitting a signal (U.S. Pat. No. 6,188,824), but these patents also provide a method of fabricating a bundle of multiple photonic crystal fibers based on a principle of a conventional total internal reflection mechanism.
In particular, U.S. Pat. No. 6,260,388 discloses a fabricating and drawing method of a photonic crystal fiber preform using a modified extrusion die so as to fabricate a honeycomb-shaped structure consisting of inorganic materials useful to fabricate a catalytic converter used in a release region of an automobile, but the modified extrusion die is different from an extrusion die of the present invention in its structure. In addition, this patent is different from the present invention in that a raw material used in the method is, for example, a mixture of glass powder and a binder, which requires a high temperature sintering process.
An extrusion die of the present invention is advantageous in that the die is useful to extrude a molten polymer, and an optical material different from the optical material used as a continuous phase can constitute the dispersed phase, instead of air.